This seizure notice was posted on the Silk Road website by the Department of Justice.

By Jonathan Dienst and Joe Valiquette, NBC News

NEW YORK -- Authorities have shut down an underground website called “Silk Road” that the FBI says served as a major marketplace for drug dealers and hackers, and have accused the owner of involvement in a possible murder-for-hire plot.

Prosecutors called Silk Road the most “sophisticated and extensive criminal marketplace on the Internet." In its two years of operation, according to prosecutors, Silk Road was used by thousands of drug dealers and hundreds of thousands of customers from across the globe. Justice Department officials said the site racked up more than $1 billion in sales and more than $80 million in commissions.

Website owner Ross William Ulbricht -- known as “Dread Pirate Roberts” -- was arrested at a library in San Francisco Tuesday by the FBI. He is accused of setting up a special network that would help drug dealers avoid detection by hiding IP addresses. Silk Road also used the internet currency known as “Bitcoins” to help facilitate the illegal deals, prosecutors said.

The drugs allegedly offered for sale included ecstasy, cannabis, opioids and stimulants, and the FBI said more than 150 listings on the site concerned computer-hacking services. According to the criminal complaint, undercover law enforcement made over 100 purchases of drugs like cocaine, LSD and heroin from Silk Road vendors.

According to the criminal complaint, Ulbricht also allegedly once tried to hire a hitman to kill a Silk Road user who had been trying to extort $500,000 from him. The user was threatening to “leak” the names of Silk Road users. Investigators said Ulbricht offered $80,000 and the name and address to an online "hitman," as well as the fact that the alleged target had a “wife + 3 kids.”

In March 2012, according to the complaint, a Silk Road vendor named "FriendlyChemist" began sending threats to Ulbricht via Silk Road's private messaging system. He said he had a long list of names and addresses of Silk Road customers and would publish the list on the web unless Ulbricht paid him $500,000, which FriendlyChemist said he needed to pay off his suppliers.

Prosecutors say that Ulbricht then began sending messages to a Silk Road user named "redandwhite" about putting a "bounty" on Friendly Chemist's head. "I wouldn't mind if he was executed," read one alleged message. Ulbricht then allegedly sent a message saying he'd had "a clean hit" done for $80,000. "Are the prices you quoted the best you can do?" asked Ulbricht, according to the complaint. "I would like this done asap as he is talking about releasing the info on Monday."

Authorities said no known shooting ever took place.

In one two-month period, the FBI said there were over 1.2 million messages exchanged through Silk Road. Investigators also said that members of the small staff that helped Ulbricht administer the website were paid between $1,000 and $2,000 per week.

Federal prosecutors have charged Ulbricht with narcotics trafficking conspiracy, computer hacking conspiracy and money laundering conspiracy. The alleged murder-for-hire plot is listed as an "overt act" as part of the narcotics trafficking conspiracy count.